App menu: standardize Help/About/Quit

Use "About" rather than "About Archive Manager"

https://wiki.gnome.org/Design/HIG/ApplicationMenus

https://bugzilla.gnome.org/show_bug.cgi?id=720142
App menu: rename Folders to Sidebar

https://bugzilla.gnome.org/show_bug.cgi?id=708655
renamed view-folders as view-sidebar in the code as well